The present invention relates to a new and improved method of, and apparatus for, detecting at least one reactive gas, particularly at least one reducing gas, and especially although not exclusively carbon monoxide in a gas mixture to be investigated, particularly in air.
In its more particular aspects, the present invention relates specifically to a new and improved method of, and apparatus for, detecting at least one reactive gas, particularly at least one reducing gas, in a gas mixture to be investigated, particularly in air, by means of determining the attenuation of the intensity of an infrared radiation beam in a gas detector which comprises an infrared radiation source, an infrared radiation detector and an electronic evaluation circuit arrangement.
Apparatuses for detecting compounds which preferably absorb electromagnetic radiation, particularly infrared radiation, at certain frequencies are generally known in the field of spectroscopy. Such apparatuses, however, are extremely expensive and, therefore, can not be employed in gas monitoring or alarm installations. For purposes of environmental protection, monitoring garages and parking installations, fire protection, explosion protection and for similar purposes gas detectors are in use for some time. Such gas detectors may comprise inexpensive gas sensors like, for example, pellistors or metal oxide semiconductors which possess an electric conductivity which changes under the action of certain gases. In order to measure the changes in conductivity which occur under the action of the gases to be detected, such gas sensors must have a predetermined minimum size and must be heated to a relatively high temperature. As a result, such gas sensors have a relatively high electric power consumption. The use of the gas sensors in gas monitoring installations is thus restricted since the eventually required emergency power generators would have to be dimensioned in order to generate a correspondingly large amount of power.
There have already been conducted experiments for utilizing the changes in optical properties of metal oxide semiconductors under the action of certain gases for detecting such gases.
It is already known in the scientific literature that, for example, rhodium which is finely distributed on alumina, when exposed to gaseous carbon monoxide, shows a very high infrared extinction coefficient in the region of the carbon monoxide band of gaseous carbon monoxide, see J. Chem. Phys. 74 (07), pages 4150 to 4155, published Apr. 1, 1981. Accordingly, the extinction coefficient in the wave number range of about 1950 to about 2150 cm.sup.-1 of Rh(CO).sub.x is about 35 times greater than the extinction coefficient of gaseous carbon monoxide. According to a publication in J. Chem. Phys. 74 (11), pages 6487 to 6497, published June 1, 1981, the compound Rh(CO).sub.x is also sufficiently stable in the presence of oxygen so that this rhodium compound can also be formed at room temperature. This effect, however, has not been utilized for the detection of reducing gases like, for example, carbon monoxide in gas monitoring installations.